1. Field of the Invention
The present invention relates to a resin composition for a foamed insulator of high-frequency co-axial cables utilized in mobile communication facilities and microwave communication facilities, and also relates to high-frequency co-axial cables using the resin composition.
2. Description of Related Art
High-frequency co-axial cables which are utilized in mobile communication facilities necessary for cellular phones and in TV stations' microwave communication facilities tend to take higher and higher service frequencies in order to increase communication rate and communication capacity. Accordingly, there is a need for high-frequency co-axial cables exhibiting a less signal attenuation. The amount of signal attenuation of the co-axial cable is the sum of conductor loss attributable to the conductor diameter and dielectric loss attributable to the insulator material (e.g., polyethylene).
However, the conductor loss cannot easily be changed because it is determined by the shape of the cable. Therefore, to reduce the amount of signal attenuation, it is necessary to reduce the dielectric loss.
Typically, the dielectric loss is expressed by the relationship shown in the following Eq. (1). Herein, the dielectric constant is denoted by ∈, dielectric tangent is denoted by tan δ, and frequency is denoted by f.Dielectric Loss ∝√{square root over (∈)}×tan δ×f  (1)
Generally, the dielectric tangent (hereafter, abbreviated as tan δ) of high-density polyethylene (HDPE) is smaller than that of low-density polyethylene (LDPE). It is considered that this is because the high-density polyethylene has fewer side chains in the molecular structure. Accordingly, the high-density polyethylene is used in many cases as a main material of resin composition for the insulator material of the co-axial cable.
Besides the tan δ of the main material, it has been proposed to decrease the dielectric loss by taking a foamed insulator which is created by chemically or physically foaming a resin composition when molding the insulator (see, e.g., JP-A 2008-027899 and JP-A 2002-251923).
However, since the high-density polyethylene has a small number of branches and its molecules do not intertwine much, the melt fracture tension (MT) is also small. For this reason, if the high-density polyethylene is used and the foaming degree of the foamed insulator is increased, bubbles do not separately disperse but are prone to coalesce to be continuous bubbles (air holes), i.e., bubbles flock together. As a result, the voltage standing wave ratio (VSWR) increases (worsens). Thus, conventionally, a low-density polyethylene having a large melt fracture tension has been frequently blended at the sacrifice of tan δ (dielectric loss).